config: handle conditional include when $GIT_DIR is not set up

If setup_git_directory() and friends have not been called,
get_git_dir() (because of includeIf.gitdir:XXX) would lead to

    die("BUG: setup_git_env called without repository");

There are two cases when a config file could be read before $GIT_DIR
is located.

The first one is check_repository_format(), where we read just the one
file $GIT_DIR/config to check if we could understand this
repository. This case should be safe. We do not parse include
directives, which can only be triggered from git_config_with_options,
but setup code uses a lower-level function. The concerned variables
should never be hidden away behind includes anyway.

The second one is triggered in check_pager_config() when we're about
to run an external git command. We might be able to find $GIT_DIR in
this case, which is exactly what read_early_config() does (and also is
what check_pager_config() uses). Conditional includes and
get_git_dir() could be triggered by the first
git_config_with_options() call there, before discover_git_directory()
is used as a fallback $GIT_DIR detection.

Detect this special "early reading" case, pass down the $GIT_DIR,
either from previous setup or detected by discover_git_directory(),
and make conditional include use it.

Noticed-by: Bert Wesarg <bert.wesarg@googlemail.com>
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

cache.h

@@ -1887,6 +1887,7 @@ enum config_origin_type {
 
 struct config_options {
 	unsigned int respect_includes : 1;
+	const char *git_dir;
 };
 
 typedef int (*config_fn_t)(const char *, const char *, void *);